<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <button>1</button>
  <button>2</button>
  <button>3</button>
  <button>4</button>
  <button>5</button>
  <script>
    const btns = document.querySelectorAll('button')

    for(var i=0;i<btns.length;i++){
     
      (function(i){
        btns[i].onclick=function($el){
          console.log($el,i)
        }
      })(i)

    }
  </script>
</body>
</html>